Repeater circuits



OGL 15, 1929 P. B, FLANDERS REPETER CIRCUITS Filed May 17, 1928 2Sheets-Sheet A w w H N am I Rw QN A m &1 ww .wlvm m El #E Q NKY M RN N\.N m `NN Ik #INN l H N R\ m AI 1 EFS w SQ Nm Tm W N u Q f w Nm m.

A TUR/VEY Oct. 15,' 1 929. R B, FLANDERS L''B REPEATER CIRCUITS FiledMay 17, 192B 2 SheetS-Shet 2 /NVE/VTUR PAUL /ANDERs v A7 @RA/EY PatentedOct.' 15, 1929 UNITED STATES PATENT OFFICE PAUL B. FLANDER-S, OF EASTORANGE, NEW JERSEY, ASSIGNOR TO BELL TELEPHONE LABORATORIES,INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK BEPEATERCIRCUITS Application led May 17, 1928. Serial No. 278,499.

This invention relates to the transmission of elect-ric currents andparticularly to circuits for repeating such currents.

rl`he invention has special reference to signal-operated repeatercircuits of the type in which. between periods of transmission, therepeater is not in condition to transmit etlicientlv in eithei`direction` but fiow of signaling currents in either direction controlsapparatus to condition the repeater for efficient transmission inthatdirection.

An object of the invention is to provide iinprovements in the two-waytransmission of electric currents` for instance. in the t-wo-wayrepeating of telephonie currents.

In accordance with the invention, a twoway signal-operated repeater ofthe type defined above provide in which efficient operation is obtainedwithout necessitating the use of movable switching elements forconditioning the repeating paths.

ln a specific embodiment, the repeater of the invention comprises twooppositely directed unilateral paths for repeating signaling currents inopposite directions between two two-wire circuits. The input of eachunilateral path is connected to its associated twowirecircuit through aconjugate connection comprising a hybrid coil transformer and balancingnetworks forming a bridge circuit, one of which networks includes theoutput impedance of a three-electrode space discharge amplifying device,which device in the following discussion will be referred to as thecontrol device. The, two bridge circuits are preferably initiallybalanced so that the transmission loss through each hybrid coil from atwo-wire circuit` to the unilateral repeating path is substantiallyinfinite, thus normally preventing transmission in either directionbetween the two-wire circuits.

'l`o enable transmission of received signaling energy between thetwo-wire circuits in a given direction, a portion of the received energvis utilized to imbalance the bridge circuit connected to the unilateralrepeating path for that direction, so as to decrease the transmissionloss through the hybrid coil portion of the bridge circuit, and allowsignaling energy to be transmitted therethrough to t-he amplifier in theassociated unilateral repeating path. This is accomplished by divertinga portion of the received signaling energy through an auxiliary cont-rolpath, amplifying and rectifying the diverted energy and impressing it onthe inputcircuit of the control device so as to va ry its outputimpedance and thus the balance of the bridge circuit including thatimpedance. To prevent the amplified energy transmitted through therepeating path for either direction from being effective to unbalancethe bridge circuit of the repeating path for the opposite direction, aportion of the rectified energy in the input of the control device inone auxiliary control circuit is utilized to neutralize the effect ofthe amplified energy incident on the other auxiliary cont-rol path.

A feature of the invention is a two-way signal-operated repeater inwhich echoes are effectively suppressed without necessitating the use.of movable Switching elements.

The invention itself in its objects and advantages will be bestunderstood by reference to the following detailed description taken inconnection with the accompanying drawings, Fig. 1 ofwhich shows t-heinvention applied to a two-way signal repeater circuit utilizingseparate amplifiers for transmission in the opposite directions, andFlg. 2 of which shows the invention applied to a two-way signal repeatercircuit utilizing av single amplifier for transmission in bothdirections.

-The two-way repeater circuit of Fig. 1 comprises two two-Waytransmission lines 1 and 2 connected through an eastwardly directedone-way circuit 3 including an amplifying device 4 and the wcstwardlydirected one-way circuit 5 including an amplifying device 6. A bridgecircuit comprising a. hybrid coil 7 having the usual series and linewindings, and impedance networks 8 and 9 is connected between theamplifying device 4 in repeating path 3 and line 1, the input of theamplifying device 4 being conneet-ed directly across the bridge pointsof the hybrid coil, and line l being connected effectively across theother diagonal of the bridge circuit by being connected to the terminalsof the series winding of hybrid coil 7. Another bridge circuitcomprising hybrid coil 10 having the usual series and line windings, andbalancing networks 11 and 12 is connected between amplifying device 6 inrepeating path 5 and line 2 the input of device 6 being connecteddirectly across the bridge points of hybrid coil 10, and line 2 beingconnected effectively across the other diagonal of the bridge circuit bybeing connected to the terminals of the series winding of hybrid coil10. The output of amplifier 4 in repeating path 3 is coupled bytransformer 13 across transmission line 2, and ,the output of amplifier6 in repeating path 5 is coupled by transformer 14 across transmissionline 1.

The balancing networks 8, 9, 11 and 12 may comprise any desiredcombination of impedance elements, but, as illustrated, each preferablycomprises a parallel combination of resistance and inductance.

Bridged across the transmission line 1 in front of the series winding ofhybrid coil 7 is an auxiliary control circuit 15 comprising a pluralityof transmission devices connected in cascade including in order, anamplifier 16, a rectifier 17, a low pass filter 18 and a control device19. The rectifier 17 may comprise one or more space discharge de` vicesbut preferably comprises two threeelectrode vacuum tubes 22 and 23connected in push-pull relation. The control device 19 preferablycomprises the three-electrode space discharge devices 20 and 21connected in push-pull at their outputs and having a portion of theirgrid circuits in common. The output of filter 18 is connected to thecommon branch of the grid circuits oftubes 20 and 21 across a resistance24, in such man`- ner that the potential variations in the output offilter 18 are applied to the two tubes symmetrically. The outputcircuits of tubes 2() and 21.are coupled to the inductive element in thebalancing network 9 by an output winding 25, so that the control device19 effectively controls the impedance of network 9 as viewed from thehybrid coil 7.

- A control circuit 15 similar to the control circuit 15 just describedand comprising elements similar to those of the latter control circuitand similarly connected, is bridged across transmission line 2 in frontof the series winding of hybrid coil 10. The elements of control circuit15 are identified by characters similar to those of the correspondingelements of circuit 15 with the addition of a prime mark. The output ofcontrol channel 15 is coupled by the output winding 25 .of controldevice 19 to the inductive element in balancing network 12, so that thecontrol device 19' will effectively control the impedance of network 12as viewed from hy rid coil 10. l

A resistance 27 in the common portion of the grid circuits of rectifier17 in control circuit 15' is connected by connections 28 across theoutput of filter 18 in control circuit 15 in such manner that theportion of the rectified current from filter 18 flowing throughresistance 27 will produce a voltage drop therein which will make thepotentials applied to the grids of tubes 22l and 23 of rectifier 17 morenegative. Similarly, a resistance 27 in the common portion of the gridcircuits of rectifier 17 in control circuit 15 is connected byconnections 28 across the output of filter 18 in control circuit 15 insuch manner that the portion of the rectified current from filter 18fiowing through resistalice 27 will produce a voltage drop therein whichwill make the potentials applied to the grids of tubes 22 and 23 ofrectifier 17 more negative.

Delay circuits 26 and 26 of suitable type are connected respectively intransmission i line 1 between the connection thereto of control circuit15 and the series winding of hybrid coil transformer 7 and intransmission line 2 between the connection thereto of control circuit 15and the series winding of hybrid coil 10, for the purpose of delayingpropagation of signaling waves from line 1 and line 2, respectively, tohybrid coils 7 and 10.

The operation of the system of Fig. 1 is as follows: The circuits ofcontrol device 19 comprising tubes 20 and 21, and the circuits ofcontrol device 19 comprising tubes 20 and 21 are initially adjusted sothat when no signaling variations are applied to the inputs of thesetubes, the output impedances of the cont-rol devices 19 and 19 are ofsuch values that the bridge circuits comprising, respec! tively, hybridcoil 7 and networks 8 and 9, and hybrid coil 1() and networks 11 and 12are balanced. For example, the output impedance of tubes 20, 21, 20 and21 maybe made infinite by adjusting the tube circuits so that no spacecurrent flows therein, as by adjusting the negative potentials on thegrids of the tubes so as to neutralize the effects of the potentialsapplied to the plates of the tubes, the values of the impedance elementsin balancing network 8 are initially relatively adjusted with respect tothe impedances of network 9 including the initial output impedance ofcontrol device 19 to balance the bridge circuit comprising hybrid coil 7and networks 8 and 9, and thus make the transmission loss through thehybrid coil 7 substantially infinite. Similarly, the values of' theimpedance elements in balancing networks 11 and 12 associated withhybrid coil 10, may be relatively adjusted so that the bridge circuitcomprising hybrid coil 10 and balancing networks 11 and 12, is initiallybalanced and the transmission loss through hybrid coil 10 is thereforesubstantially infinite. This means that in the absence of llOtransmission in one direction in the system, the repeating pat-h forthat direction is inoperative to repeat signaling variations betweentransmission lines 1 and 2.

Now, if signaling variations are received over transmission line 1 inthe direction from west to east, a portion of these variations will bediverted from line 1 through control circuit 15, amplified in amplifier16 and rectified in rectifier 17. The rectified variations which passthrough the low pass filter 18 are applied symmetrically to the grids oftubes 2() and 21 of control device 19 across resistance 24. The appliedvariations will cause the potentials'on the grids of tubes 20 and 21 torise to such value as to make space current flow in each of these tubes.The resulting output currents of tubes and 21 will flow through outputwinding in the same 2o direction and reduce the plate impedances ofthese tubes from an infinite value to a finite value, thus decreasingthe effective impedance of network 9 as viewed from the hybrid coil 7.The bridge circuit comprising hybrid 25 coil 7 and networks 8 and 9,therefore, will be unbalanced, so as to decrease the transmission lossthrough hybrid coil 7 from an infinite value to afinite value. Thesignaling energy from line 1, the transmission of which to hybrid coil 7has been delayed for the interval of time necessary for operation ofcontrol device 19 by proper design of delay circuit 26 then will betransmitted through hybrid coil 7 to amplifier 4 in repeating path 3.The amplified variations in the out-put of amplifying device 4 areimpressed by means of transformer 13 upon the outgoing transmission line2. As the bridge circuit comprising hybrid coil 10 and networks 11 and12 coupling the input of the oppositely directed repeating path 5 totransmission line 2, is normally balanced, substantially no part of theamplified variations will be transmittedA through the hybrid coil 10into the repeating path 5. However, a portion of the amplifiedvariations transmitted through transformer 13 will be diverted intocontrol circuit 15 and amplified by amplifier 16. The resistance 27 inthe common portion of the grid circuits of rectifier 17 is of propervalue, so that the portion of the rectified variations transmittedthereto from the output of filter 18 in control circuit 15 overconnections 28 will produce a voltage drop in resistance 27 of suchmagnitude as to neutralize the effect of' the amplified variations inthe output of amplifier 16 on the grids of rectifier 17. The rectifier17', therefore, will remain in its normal condition, and the amplifiedvariations impressed thereon will be prevented from unbalancing thebridge circuit comprising hybrid coil '10 and networks 11 and 12.

When the transmission of signaling variations from west to east overtransmission line 65 1 ceases, tubes 20 and 21 of control circuit 19will return to their normal condition, and the bridge circuit comprisinghybrid coil 7 and networks 8 and 9, will again become balanced so thatthe loss through hybrid coil 7 is again substantially infinite. Thedelay circuit 2G, of course, is relatively designed with respect tocontrol circuit- 15 so that all received signaling energ f from line 1is transmitted through hybrid coil 7 before the bridge circuit becomesunbalanced.

The operation of the system when transmission is in the direction fromeast to west is similar to that when the transmission is in thedirection from west to eastand therefore need not be described.

The push-pull connection of tubes 20 and 2l of control device 19 incontrol channel 15 and of tubes 20 and 21 of control device 19' incontrol circuit 15', is for the. purpose of preventing the introductionof' even harmonics of the signal waves into the main repeating pathsbecause of the action ot tubes 20 and 21 or tubes 20 and 21, on path 3and 5, respectively. A single tube may, however, be used in the controldevices instead of the push-pull tubes, in which case. if the circuit ofcach control tube is so adjusted that its plate resistance is reduced toits lowest value by the control energy impressed on the input ot thetube, the variation of the plate resistance with the impressedalternating voltage will be small, and the harmonics introduced by thecontrol circuits in the main repeat-ing paths will not causeobjectionable interference therein.

The purpose of low pass filters 18 and 18 in control circuits 15 and15', respectively, is to filter out large variations in the controlcurrents which might cause difliculty in obtaining smooth operation ofthe conl rol devices in unbalancing t-he bridge circuits.

The elements in the control circuits 15 and 15 and the hybrid coiltransformers 7 and 10 should be so selected that the loss through thehybrid coils will not vary sensibly with variations in amplitude of thereceived signaling variations. One way of aiding this would be to usefor the rectifier tubes 22, 23 in control circuit 15, and for therectifier tubes 22', 23' in control circuit 15', respectively, tub'eswhich have such a saturation characteristic that received waves above agiven amplitude value will not cause any increase in the amplitude. ofthe control waves applied t-o the cont-rol devices in the two controlcircuits. f

Fig. 2 shows a modification of the two-way repeat-ing system of Fig. 1,in which a single amplifier 50 is employed for repeating signalingenergy between the two-way transn'lission lines 51 and 52 in bothdirections.. A bridge circuit comprising a hybrid coil transformer 53having the usual series and line windings, and balancing networks 54 and55, is connected between amplifier 5() and transmission line 51, theinput of amplifier 50 being connected directly across the bridge pointsbetween the line windings of hybrid coil 53, and line 51 being connectedeffectively across the other diagonal of the bridge circuit by beingconnected to the terminals of the series winding of hybrid coil 53. Asecond brid e circuit comprising hybrid coil 53 having t 1e usual seriesand line windings, and balancing networks 54 and 55 is connected in asimilar manner between the input of amplifier 50 and transmission line52.

A third bridge circuit comprising a hybrid coil 56 and balancingnetworks 57 and 58 is connected between the output of amplifier 50 andtransmission line 52, the outputl of the amplifier 50 being connectedeffectively across one diagonal of the bridge .circuit by beingconnected to the terminals of the series winding of hybrid coil 56, andtransmission line 52 being connected directly across the other diagonalof the bridge circuit at the bridge points between the line windings ofhybrid coil 56. In similar fashion, a fourth bridge circuit comprisingthe hybrid coil 56', and balancing networks 57 and 58 is connectedbetween the output of amplifier 50 and transmission line 5 The balancingnetworks 54, 55, 54, 55', 57, 58, 57 and 58 ma comprise any desiredcombination of impecances, but preferably each comprises, asillustrated, a parallel combination of resistance and inductance.

Bridged across the transmission line 5l in front of the series windingof hybrid coil 53 is a control circuit 59 including a plurality oftransmission elements comprising in order an amplifier 60, a rectifier61, a low pass filter 64 and the controlldevices 65 and 66. The controldevice 65 preferably comprises two three-electrode space dischargedevices 67 and 68 connected in push-pull relation at their outputs andhaving a portion of theirgrid circuits in common. Similarly, the controldevice 66, preferably comprises two threeelectrode space dischargedevices 71 and 72 connected in push-pull relation at their outputs andhaving a portion of their grid circuits in common.

The output of filter 64 is connected to the common branch of the gridcircuits of tubes 67 and 68 of control device 65 across a resistance 69,and to-the common branch of the grid circuits of tubes 71 and 72 ofcontrol device 66 across a resistance 73, in such manner that anypotential variations in the output of filter 64 will be appliedsymmetrically to the inputs oftubes 67 and 68, and to the inputs oftubes 7l and 72. The outputs of tubes 67 and 68 of control device 65 arecoupled by means of output winding 70 to the inductlve element innetwork 55, so that control device 65 effectively controls the impedanceof network 55 as viewed from hybrid coil 53. Similarly, the outputs oftubes 71 and 72 of control device 66 are coupled by means of outputwinding 74 to the inductive element in impedance network 58 so thatcontrol device 66 effectively controls the'impedance of network 58 asviewed from .hybrid coil 56. The rectifier 61 may comprise one or morespace discharge devices, but preferably comprises the twothree-electrode space discharge devices 62 and 63 connected inI pushpullrelation.

A control circuit 59 similar to the control circuit 59 just describedand comprising elements similar to those of the latter circuit andsimilarly connected, is bridge across transmission line 52 in front ofthe series Windlng of hybrid coil 53. The elements of control circuit 65are identified by characters similar to those of the correspondingelements in control circuit 65, but with the addition of a prime mark.The output of control device 65 in control channel 59 is coupled byoutput Winding 70 to the inductive element in balancing network 55', andthe output of the control device 66 is connected by output winding 74 tothe inductive element in balancing network 58', so that control devices65 and 66 respectively, control the impedance of networks 55 and 58 asViewed from hybrid coils 53 and 56', respectively.

A resistance 75 in the common portion of the grid circuits of tubes 62and 63 of rectifier 61 in control circuits 59 is connected by connection76 across the output of filter 64 in control circuit 59 in such manner,that the portion of the rectified current from filter 64 flowing throughresistance 75 will produce a voltage drop therein which will make thepotentials applied to the grids of tubes 62 and 63 of rectifier 61 morenegative. Similarl a resistance 75 in the common portion of tiie gridcircuits of tubes 62 and 63 of rectifier 61 in control channel 59 isconnected .by connections 76 across the output of filter 64 in controlchannel 59', in such manner that the rectified currents from filter 64fiowing through resistance 75 will produce a voltage drop therein whichwill make the potentials applied to the grids of tubes 62 and 63 ofrectifier 61 more negative.

Delay circuits 77 and 78 of any suitable type are connected,respectively, in transmission line 51 between the connection thereto ofcontrol circuit 59 and the series winding of hybrid coil 53,1and intransmission line 52 between the connection thereto of control circuit59 and the series. winding of hybrid coil transformer 53', for thepurpose of dela ing the propagation of signaling currents fyiom line 1and line 2, respectively, to hybrid coil 53 and 53.

The loperation of the system of Fig. 2 will be clear from thedescription of its operation in transmission from west to east, as theoperation in transmission from east to west is similar.

The circuits of control device 65, comprising tubes 67 and 68, ofcontrol device 66 comprising tubes 71 and 7 2, of control device 65comprising tubes 67 and 68 and a control device 66 comprising tubes 7land 72', are initially adjusted so that when no signaling variations areapplied to the inputs of these tubes, their` output impedances are suchthat the bridge circuits associated therewith are balanced; and thetransmission losses through hybrid coils 53, 56, 53 and 56, respectivelyare, therefore, substantially infinite. This may be accomplished in amanner similar to that described in connection with hybrid coils 7 and10 in the system of Fig. 1. This means that in the absence oftransmission in either .direction in the system, the repeating path forthat direction is inoperative to transmit signaling variations betweentransmission lines 5l and 52.

Now, it' signaling variations are received over transmission line 51 inthe direction from west to east, ay porton of these variations will hediverted from line 51 through control circuit 59, amplified in amplifier60 and rectified in rectifier 61. The portion of the rectifiedvariations which is transmitted through low pass filter 64 is impressedon the input circuits ot control devices 65 and 66 in parallel crossresistances 69 and 73. The impressed variations are effective to raisethe potentials applied to the grids of the tubes of control devices 65and 66, respectively, to such value as to make space current flow in thetubes thereof. The resulting output currents in tubes 67 and 68 ofrectifier 65 will flow through the output winding 70 in the samedirection and reduce the plate impedances of these tubes from aninfinite value to a finite value, thus decreasing the impedance ofnetwork 55 as viewed from hybrid coil 53. The normally balanced bridgecircuit comprising hybrid coil 53 and networks 54 and 55, Willtherefore, be unlmlanced so as to decrease the transmission loss throughhybrid coils 53 from an infinite to a finite value. The signalingvariations from transmission line 51, which have been delayed for theinterval of time required for unbalancing hybrid coil 53 by'preperdesign of delay circuit 77, therefore will pass through hybrid coil 53to the input of amplifier 50 and be amplified therein.

Similarly, the output currents of tubes 71 and 72 ot control device 66,due to the rectified signaling variations impressed symmetriallly ontheir input circuits across resistance 78 will flow in the samedirection through output coil 7l and reduce the plate impedances oftubes 7l and 72 from an innite value to a finite value, thus effectivelydecreasing the impedance ot that network as viewed from hybrid coil 56.This will result in the unbalancing of the bridge circuit comprisinghybrid coil 56 and networks 57 and 58, and a decrease in thetransmission loss through hybrid coil 56 from an infinite to a finitevalue allowing the amplified variations in the output of amplifier 50 tobe transmitted through hybrid coil 56 to the outgoing line 52.

Because the bridge circuits comprising, `respectively', hybrid coil 53and balancing networks 51 and 55', and hybrid coil 56 and balancingnetworks 57 and 58. are normally balanced, the amplified variations fromthe output of amplifier 50 cannot be transmitted through hybrid coil 53to the input of amplifier 50, or through hybrid coil 56 to transmissionline 51. However, a portion of the amplified variations transmittedthrough hybrid coil 56 will be diverted into control circuit 59 andamplified in amplifier 60. The resistance 75 in the common portion ofthe grid circuits of rectifier 61 is of proper value, so that thevoltage drop therein due to the portion of the rectified variations fromthe output of filter 64 transmitted thereto over connect ions 76, willbe of such magnitude as to neutralize the effect on the grids of' thetubes of rectifier 61 of the amplified variations in the output ofamplifier The imbalance of the. bridge circuits including hybrid coil 53and the bridge circuit including hybrid coil 56 by the amplifiedvariations from the output of amplifier 50 is thereby prevented.

As in the case of' the control devices in the system ot' Fig. 1, thepush-pull connection of the outputs of the tubes in control devices 65,66, 65 and 66 is for the purpose of preventing thc introduction of evenharmonics of the signal waves. into the repeating paths, but singletubes may be used in place of the push-pull tubes if the circuitsthereof are adj usted in the manner described in connection with thesystem of Fig. 1, so that the variations of the plate resistanees withthe alternating voltages impressed on their input circuits are small.Low pass filters 64 and 6i are for the purpose of filtering out largevariations in the control potentials applied to the inputs of controldevices 65, 66, 65 and 66.

Vhile the invention has been described as applied to certain typicalsystems including certain particular combination of apparatus, it is tobe understood that it is not limited to the systems or the particularcombinations of apparatus described, but only by the scope of theappended claims.

W'hat is claimed is:

1. A transmission system comprising two two-way transmission circuits, apath for repeating signaling waves between said twoway circuits, abridge circuit connected in said repeating path, said bridge circuitbeing normally balanced so as to effectively prevent transmissionbetween said two-way circuits over said repeating path, and meansresponsive to transmission of signaling waves in one of said two-waycircuits for unbalancing said bridge circuit so as to enabletransmission of said signaling waves between said two-way circuits oversaid repeating path.

2. A transmission system comprising two two-way transmission circuits,two oppositely directed unilateral paths adapted for repeating signalingwaves in opposite directions between said two-way circuits, a bridgecircuit connected between the input of each unilateral path and adifferent one of said two-way circuits and normally balanced so as toe'ectively prevent transmission between said two-way circuits in eitherdirection, and means responsive to the transmission of signaling wavesin said system in one direction for unbalancing one of said bridgecircuits to allow transmission of signaling waves between said two-waycircuits over the repeating path for said one direction.

3. A transmission system in accordance with claim 2, and in which eachof the balanced bridge circuits comprises a hybrid coil transformer andassociated balancing impedance networks, and said means for unbalancingsaid one of said bridge circuits comprises means responsive totransmission in said system for varying the impedance of one of saidnetworks.

4. A transmission system in accordance with claim 2 and in which each ofsaid balanced bridge circuits comprises the windings of a hybrid coiltransformer and associated balancing impedance networks, one of saidimpedance networks comprising the output circuit of a space dischargedevice, and said means for unbalancing one of said bridge circuitscomprises means or varying the impedance of said output circuit of saidspace discharge device.

5. A transmission system in accordance with claim 2 and in which each ofsaid balanced bridge circuits has for its respective arms the windingsof a hybrid coil transformer, a constant impedance device and a variableimpedance device, and the means' for unbalancing said bridge circuitcomprises means responsive to transmission in said system forcontrolling the impedance of said variable impedance device.

6. A transmission system in accordance with claim 2 and in which theinput of one unilateral path is effectively connected across onediagonal of one of said bridge circuits, and one of said two-waycircuits and the output of the other of said unilateral paths connectedeffectively across the other diagonal of said balanced bridge circuit.

7. A transmission -system in accordance with claim 2 and in which eachof said bridge circuits includes in one of its arms the eiective outputimpedance of a space discharge device having an input circuit, and saidmeans for unbalancing said bridge circuit comprises means for applying aportion of the signaling currents transmitted over one of said twowaycircuits to said bridge circuit, to the input circuit of said spacedischarge device so as to change the effective output impedance thereof.

8. A repeating system comprising two twoway transmission circuits, twooppositely directed unilateral paths including amplifying means, meansfor effectively connecting each two-wire circuit with the input of oneof said unilateral paths and the output of the other unilateral path,said means including a bridge circuit coupling the input of eachunilateral path with the two wire circuit, each of the bridge circuitsbeing normally balanced so as to normally prevent transmission ofsignaling variations between said two-wire circuits, means connected toeach two-wire circuit and responsive to transmission in one directiontherein for unbalancing the bridge circuit coupled thereto so as toallow transmission of signaling energy from one twowire circuit to theother over the unilateral path for said one direction, and means toprevent the transmitted energy from unbalancing the bridge circuitconnected to said other twowire circuit.

9. A repeating system, comprising a repeat- -er path between two linesections, said path containing impedance balancing means normallybalanced to prevent transmission through the repeater path, and meanscontrolled by signal waves received over a line section for upsettingthe impedance balance of said irst means and thereby rendering therepeater path operative to repeat said signals into the opposite linesection.

In witness whereof, I hereunto subscribe my name this 16th day of May,1928.

PAUL B. FLANDERS.

